Dual-555 door alarm

How it works

Closing S1 powers the circuit. Opening the normally closed S2 starts an exit interval set by R1 and C1. A later door opening longer than the second timing interval causes the horn output to activate.

The archive quotes approximately five seconds to exit and a two-second door interval. Resistor and capacitor tolerances affect both; measure the actual timing on the assembled circuit.

On the bench

Verify the reset and disarm behavior with an indicator before connecting a horn. Vehicle loads need suitable drive, fusing, and transient protection beyond a successful bench demonstration.

Resources & provenance

Rewritten from the original circuits/sec/mn3qo9c_sec004.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.